1. Field of the Invention
The present invention relates to an insulated power supply apparatus for a power conversion apparatus including series connections each having an upper arm switching element and a lower arm switching element connected in series to each other.
2. Description of Related Art
Japanese Patent Application Laid-open No. H11-178356 describes an insulated power supply apparatus for supplying a drive voltage to switching elements (IGBTs). This power supply apparatus includes transformers, a transistor for making and breaking electrical connection between the primary coils of the transformers and a DC power source, and a control circuit. The control circuit controls the transistor based on the output voltage of the secondary coil of one of the transformers.
The output voltage of the secondary coil of each of the transformers decreases with the increase of a charging current supplied to the gate of the switching element. This is because the voltage drop within the secondary coil due to a DC resistance and the like increases with the increase of the charging current. In a case where the transistor is controlled such that the output voltage of a specific one of the secondary coils, the charging current outputted therefrom being the smallest of all the secondary coils, is feedback-controlled to a target voltage, there is a concern that the conduction losses of the switching elements may become excessively large for the reasons described below.
In the above case, the output voltage of any secondary coil other than the specific secondary coil is lower than the output voltage of this specific secondary coil. Accordingly, when the switching elements whose gates are connected to the secondary coils other than the specific secondary coil are switched from the off state to the on state, their gate voltages may drop beyond the scope of the design assumption. Therefore, since the collector-emitter saturation voltages of the switching elements increase, the conduction losses of the switching elements may increase excessively.